Comments on "Hall Effect Imaging" [with reply]

Recently, Wen, Shah, and Balaban (The Encyclopedia of Physics, 2nd ed., New York, VCH, 1990) presented two novel and elegant methods for imaging electrical conductivity. In their first method, they placed a conducting sample in a steady magnetic field and applied an ultrasound pulse. The resulting motion of the conductor in the magnetic field produced a measurable voltage. In their second method, they applied a high-frequency voltage to a conducting sample in a steady magnetic field and recorded the ultrasound signal. Here, Roth and Wikswo make two points about this work: 1) The "Hall effect" is not the physical basis for Wen et al.'s techniques, and 2) their work has close experimental and theoretical connections to previous studies of magnetoacoustic imaging. In their reply, disagreeing with Roth et al., Wen and Balaban say that they did not propose two imaging methods. Their paper presented two realizations, the forward and reverse modes, of the same imaging method. They are the reciprocal versions of the same linear electrodynamic process, namely, the conversion between electrical energy and mechanical energy by the Lorentz force. In reply to Roth et al.'s statement that "the name "Hall effect imaging" is misleading" Wen and Balaban say that they extended the idea of the classical Hall effect to describe "Hall-effect imaging" because the initial motion of the charges is not driven by an electric field but by direct mechanical force.